Quick questions on Mass spectrometry: WACE Year 12 Chemistry

The tallest peak in the spectrum is the base peak, corresponding to the most stable (most abundant) fragment ion. Its abundance is set to 100 percent and other peaks are scaled relative to it. A particularly stable fragment dominates the spectrum.

Beyond the molar mass and fragments, the pattern of peaks around the molecular ion can reveal which elements are present. A small peak one unit above the molecular ion (the $\text{M}+1$ peak) comes from molecules containing a carbon-13 atom, and its relative size hints at how many carbon atoms the molecule has. More strikingly, elements with two abundant isotopes leave a characteristic doublet: chlorine gives molecular-ion peaks two units apart in roughly a $3:1$ ratio (chlorine-35 to chlorine-37), and bromine gives a near $1:1$ doublet two units apart (bromine-79 and bromine-81). Spotting one of these patterns immediately tells you the compound contains chlorine or bromine, which is a quick, high-value deduction in an unknown-identification question.